The demands of pregnancy bring about physiological changes in animals and humans alike. One of those adaptations involves the metabolic process, and a team of researchers recently set out to determine how insulin sensitivity and glucose levels differ in pregnant mares from nonpregnant mares.

A team of researchers at the Virginia-Maryland Regional College of Veterinary Medicine examined 32 healthy pasture-fed Thoroughbred mares, 22 of which were in foal and 10 of which were not.

The researchers performed an intravenous glucose tolerance test to measure insulin sensitivity, acute insulin response to glucose, and glucose effectiveness when the pregnant mares were between 25 and 31 weeks of gestation (Period 1).

The mares were then divided into two groups--both of which included pregnant and nonpregnant mares. One group was fed a high-starch supplement (39% starch; starches are known to induce a high glycemic response) and the other a high-fat/fiber supplement (14% fat/70% fiber, which elicits a lower glycemic response). Further, the researchers collected hourly blood samples from a subset of 12 mares over a 24-hour period to measure insulin and glucose levels in response to ingesting the feed supplements. Insulin sensitivity and other measures of glucose and insulin dynamics were again assessed at 47 weeks of gestation (Period 2).

The test results showed:

During Period 1 the pregnant mares had lower insulin sensitivity and glucose effectiveness and higher acute insulin response to glucose than the nonpregnant mares;

From Period 1 to Period 2, insulin sensitivity decreased in the nonpregnant mares; and

Pregnant mares fed a high-starch diet had significantly higher glycemic and insulinemic responses compared to nonpregnant mares fed the same feed or pregnant and nonpregnant mares fed the high-fat/fiber feed.

"Mares in late pregnancy are insulin resistant relative to nonpregnant mares, and post-feeding blood insulin responses are greatly exaggerated in pregnant versus nonpregnant mares for feeds rich in nonstructural carbohydrates (mainly starch in the case of the feed used in this study)," explained Raymond J. Geor, BVSc, PhD, Dipl. ACVIM, professor and chairman of Large Animal Clinical Sciences at Michigan State University, and author on the study.

He also explained that the exaggerated post-feeding insulin responses could indicate that some pregnant mares are at risk for laminitis, as high insulin concentration might be a risk factor for that condition.

"Careful attention to diet is particularly important for pregnant mares with a history of metabolic syndrome-associated laminitis, with control of dietary nonstructural carbohydrates (e.g. restricted use of grain-based feeds)," Geor suggested to owners. "As well, avoid overfeeding that causes or exacerbates obesity, because this will also worsen insulin resistance and hyperinsulinemia."

This study, "Evaluation of the effects of pregnancy on insulin sensitivity, insulin secretion, and glucose dynamics in Thoroughbred mares," was published in the May 2011 issue of the American Journal of Veterinary Research. The abstract is available on PubMed.

About the Author

Freelance journalist Natalie DeFee Mendik is a multiple American Horse Publications editorial and graphics awards winner specializing in equestrian media. She holds an MA in English from Colorado State University and an International Federation of Journalists' International press card, and is a member of the International Alliance of Equestrian Journalists. With over three decades of horse experience, Natalie’s main equine interests are dressage and vaulting. Having lived and ridden in England, Switzerland, and various parts of the United States, Natalie currently resides in Colorado with her husband and two girls.

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